Recently I had to do something that sounds very simple to any good SysAdmin:
create a disk image with a booting Debian installation, from a script, with no
human interaction. The idea is to later install our software on it. Those who
want to test out soft would just need to download the image and boot it in any
virtual machine they have: qemu, virtualbox[1], you name it.

So the process could be thought as this: create a disk image, partition it,
install Debian, install a bootloader, profit! Let's try to tackle them
separately, looking at different approaches[2]:

A disk image is simply a file big enough: 1GiB, 10GiB, whatever you want. A
string of 1Gi of 0s should be enough:

dd bs=$((1024*1024)) count=1024if=/dev/zero of=stable.img

Now, that file is using 1GiB of space, but we're not sure if we're going to use
it all, and so is kinda a waste of space. Luckly, Linux is able to handle sparse
files: files that do not reserve
all the file system blocks would normally be needed, only those where data is
written. So for instance, a way to create a 1GiB (almost) empty sparse file is
this:

dd bs=1 count=1 seek=$((1024*1024*1024))if=/dev/zero of=stable.img

That is, we write a 0 at the end of a 1GiB file[3], but even if the file is so
big, it's actually using one file system block (4096 bytes, according to
dumpe2fs).

A simpler, or maybe more-intuitive-when-you-read-it[4] alternative is to use a
tool that comes in qemu-utils[A]:

qemu-img create -f raw stable.img 1G

That was easy. Now, how do we partition it? The first answer it's obvious,
namely,
fdisk, but it is not scriptable. So we look for alternatives, and one that
comes to mind is parted: it is designed with scriptability in mind, it should
be perfect!

Almost. parted needs a partition table signature in the MBR[5] and it has no
way to create one. This is at least surprising, but a little more (ab)use of
dd can save the day. It's just a matter of writing the bytes 0x55 0xaa[8] in
the last two bytes of the first sector of the image. A disk sector, up to
recently, is just 512 bytes, so:

But this will rise a problem that I'll mention later, at its proper time. So
instead, and given that we're gonna install another package anyways, we're gonna
use sfdisk[6][7]:

sfdisk -D stable.img <<EOF,,L,*;;;EOF

The next step is to format the partition inside the image. If this were a
partition image we could simply apply mkfs.ext2 (or whatver filesystem type
you want) to the file, because the filesystem would start from the beginning of
the file. But as this is a disk image, the partition starts at an offset from
the beginning:

The 0+ in the third column tells us that the partition doesn't start exactly
in the cylinder 0. That would mean it starts where the MBR is. Actually it
starts in the cylinder 0 but in the second head. According to CHS reported by
sfdisk, there are 63 sectors per track, so we just need to skip so many bytes:
63x512=32256. Coincidentally, 130- in the fourth column means that the
partition does not reach the end of cylinder 130, which is exactly what parted
was complaining above[9].

To fix the aligment we will have to do it the other way around: instead of
discovering the CHS from the image size, we'll compute the image size from some
desired CHS and a minimum image size. This can be done as such:

So we will have to somehow tell to mkfs.ext2 about the partition offset inside
the disk image. We can use something that we have been using unknowingly: loopback
devices. Who hasn't mounted an ISO-9660 image in the past? We used something like
this:

mount -o loop debian-505-i386-CD-1.iso /mnt

This is more or less equivalent to:

losetup /dev/loop0 debian-505-i386-CD-1.iso
mount /dev/loop0 /mnt

Good thing is, we can tell losetup to simulate the start of the device some
bytes inside the file. And given that everything is a file in Linux, we can even
chain loop devices, such as:

losetup /dev/loop0 stable.img
losetup -o 32256/dev/loop1 /dev/loop0

Now /dev/loop0 points to the disk image and /dev/loop1 points to the
partition. There's really not much option here, so we skip to the formatting
part, which is even more straightforward:

mkfs.ext2 /dev/loop1

Now to install Debian in this beast. Here we won't be exploring much either, but
I will explain a couple of tricks I learned to complete this task successfully.
The tool of choice is debbootstrap, which is able to install packages in a
directory as if it where the root partition, so we will need to mount it first:

mount /dev/loop1 mnt

In my case I will need to install several packages besides the base install:

Notice that the base set of packages does not include nor a kernel or a boot
loader, because this is normally installed by Debian Installer, so I added
them to the list of packages. But this is not the only thing that the installer
does (and that there is no way to repeat besides by hand): it also sets up the
environment, users, apt config (from the ones used to install) and more. We will
have to set those by hand.

Before running anything else, which will run under chroot, we will
need to also setup some of the virtual filesystems that are running on a normal
GNU/Linux setup; namely, /dev, /dev/pts and /proc. We will reuse the
host's ones, using the hability to mount a dir in another:

# aptecho"deb http://http.us.debian.org/debian stable main"> mnt/etc/apt/sources.list
echo"deb http://security.debian.org stable/updates main">> mnt/etc/apt/sources.list
# otherwise perl complains during installation that it can't set the locale# actually we will have to do some little more than just this; see belowecho"en_US.UTF-8 UTF-8"> mnt/etc/locale.gen
# when installing the kernel, if this setting is not present, it thinks the# bootloader is not able to handle initrd images[10]echo"do_initrd = Yes"> mnt/etc/kernel-img.conf

The last step is to install a bootloader. Here we have several options. lilo was
the first Linux bootloader, which was started in 1992. Even if it can
bootload almost any operating system in lots of filesystems, one of its
main drawbacks is it staticness: it reads a config file, compiles the bootloader
and installs it. After that you can't change anything (except for adding more
boot parameters to the kernel), so if you wrote something wrong and your system
does not boot, it's hard to recover. Also, if you change anything in the config file,
you have to compile and install the bootloader again.

The second and third options are the two
flavors of grub, the GNU GRand Unified Bootloader. The first iteration of grub,
grub1 or grub-legacy how it is called now, is no longer under development or
support, but a lot of people still use it for its simplicity and power. First
developed in 1999, it has the hability to read the config file at boot time and
it lets edit it and read the filesystems before booting. Its successor, grub2
or grub-pc, is even more modular and flexible, but takes time to relearn it.

Even with this last two options, I couldn't managed to reliably get a booting
image. To be fair, I managed to do it with grub-pc, but my script had to work
in a machine that boots with grub-legacy. Installing both at the same time is
impossible, and I need to use the host's bootloader because I can't reliably
fake the devices in a chrooted environment and using any virtual machine was
imposible because the image doesn't boot yet! Talk about chicken and eggs...
For the record, here's how I managed to make it work with grub-pc:

So, I needed to find a bootloader that could be installed in the host machine
without changing the actual bootloader in use. Luckily I talked to a friend
sysadmin/guru, Ignacio Sánchez, which pointed me to extlinux, which is part
of the syslinux family of bootloaders. This family also includes isolinux,
famously known for booting the iso images of most of the distributions for years.
I knew about the latter two, and I even used syslinux in a company I worked for
two years ago in a floppy disk (!!!) used to boot the old firewall[12] and another
set of diskettes for two diskless thin clients. extlinux is
the youngest of the family, which is able to read and boot from extX partitions.
The config file looks like a very simple lilo.conf:

Installing it actually consists of two steps: first installing MBR code that
boots from the partition marked as bootable[11]. The syslinux family comes with
such a MBR code, so we use it:

ddif=/usr/lib/extlinux/mbr.bin of=stable.img conv=notrunc

We're almost there. Installing extlinux is really straightforward:

extlinux --heads $heads--sectors $sectors--install mnt/boot/syslinux

It only rests to umount and dismantle the loop devices in the reverse order:

umount mnt/
losetup -d /dev/loop1
losetup -d /dev/loop0

Sometimes you need to wait a couple of seconds between these commands, because
they seem to be asynchronous. Otherwise you'll get errors that the device is
still busy, because the previous command has finished, but the async process in
the kernel has not.

The image as it is is bootable with qemu and virtualbox, but if you want to
make it bootable in other, closed virtual machines, you must convert it to vmdk.
qemu-utils to the rescue again:

qemu-img convert -O vmdk stable.img stable.vmdk

I have lots of things more to mention, but this post has got long enough as it
is. Mostly they were references to the sites I got info from, but I know that if
I try to clean it up I will procrastinate it for another month or so and probably
forget about it.

[1] Currently this needs an image conversion.

[2] Of course, I strongly recommend to check the manpages of the mentioned
tools.

[3] Quick, which is the actual size of the file? You can answer with powers of 2
if it makes it easier for you :)

[4] I have the tendency to write as-understandable-as-possible code; that means,
I know that I'll have to read and try to understand it 6 months after I wrote
it, soI try to
make it as readable as possible. That includes using long options when I
invoke tools in scripts and, of course, sensible class and variable names.

[5] Master Boot Record, the first sector in a disk.

[6] You will have to read sfdisk's manpage to understand what's all that.

[7] sfdisk has a neat trick: you can dump the partition table from one disk
and pipe it to a sfdisk affecting another disk, actually copying the
partition scheme. It comes very handy when adding disks to a raid setup.

[8] Technically we're marking it as a MSDOS type partition table.

[9] Notice that it only complains about the end bound, not the beginning bound.

[10] One interesting note: even if above I told debbootstrap to install a
Linux kernel, it actually hasn't. The package linux-image-2.6-686 is a
virtual one, and debbootstrap seems to not resolve this ones, but it
doesn't complain either.

[11] See that Boot column in the output of sfdisk at the beginning of the
post? And the * in the first and only partition? That shows it as bootable.
This is an old relic from the times when operating systems relied on a dumb
MBR code to boot. And now we're using exactly that to load a bootloader.

[12] Really
old; we're talking about a Cyrix 486DX2 at 50MHz with 16MB de RAM, 4 NICs,
all of them ISA, two of them still donning 10Base2 connectors and configurable
via jumpers. We really didn't need anything bigger since the ADSL line was
merely 2.5Mib/s.